Method and arrangement for controlling a loop filter

Abstract: The invention relates to a method and an arrangement for controlling a loop filter of a digital phase lock, the loop filter filtering a difference signal, which comes from a phase comparator at a predetermined bandwidth and is proportional to a phase error. To reduce oscillation in the adjusting method and to eliminate the errors caused by noise, the loop filter is adjusted non-linearly on the basis of the difference signal from the phase comparator in such a manner that the bandwidth of the loop filter changes.

Description: The invention relates to a method for controlling a loop filter of a digital phase lock, the loop filter filtering a difference signal, which comes from a phase comparator at a predetermined bandwidth and is proportional to a phase error. The object of a phase locking circuit is to lock to a phase of an incoming signal. Digital phase locks generally comprise a phase comparator which generates a difference voltage proportional to a phase error, i.e. the phase difference between the phase of the phase lock circuit and the incoming signal. The difference voltage is filtered by a loop filter to eliminate noise. Signals are received from the output of the loop filter for controlling a clock in such a way that the phase of the clock signal is advanced or retarded. In standard step correctors, the clock phase is corrected in standard steps upward or downward. A phase lock operating on this advance/retard principle is called lead/lag phase lock. Digital phase locks operating on lead/lag principle use digital filters of a type known as sequential filters, as loop filters . Figure 1 shows schematically the structure of a digital phase lock operating on the lead/lag principle. The phase lock comprises a phase comparator 10, a loop filter 11, a clock 12, a local oscillator 14 and a divider 13. Lead or lag pulses are generated in the phase comparator 10 according to whether the phase of the local oscillator is ahead or behind compared with an incoming signal to the comparator 10. Errors caused by noise can be eliminated from a phase correction signal by means of the loop filter 11. An output signal of the filter controls a clock signal of the phase-locked loop in such a way that its phase is advanced or retarded by removing or adding samples, respectively. Figure 2a illustrates the structure of a conventional loop filter of a lead/lag phase lock. The filter comprises three counters 20, 21, 22 and two OR gates 23, 24. Lead and lag pulses are counted in the filter by separate N counters 20 and 21. In addition to this, both pulses are brought via the OR gate 23 to a common M counter 22. If one of the N counters 20, 21 fills up before the common M counter 22, a lead pulse 25 or a lag pulse 26, respectively, is sent from the filter, depending on which one of the counters filled up. After the pulse has been sent, all counters are set to zero by means of a signal 27. On the other hand, if the common M counter 22 fills up at first, all counters are set to zero by the signal 27 and no correction is made. The lengths of the counters are selected in such a way that N < M < 2N. The bandwidth and correction rate of the filter can thus be influenced by the values of the numbers N and M. An advantage of this filter is good attenuation in loop operation, but a drawback is slow correction. Another conventional manner of filtering a phase correction signal is presented in Figure 2b. The filter in question is a conventional digital Infinite Impulse Response (IIR) filter comprising two amplifiers 30, 31, a summer 32 and a delay element 33, followed by a threshold detector 34. The integration time of the filter is selected by the parameters 30 and 31 of the filter. The threshold detector operates as follows: if input signal of detector ≥ threshold, output is + 1 if absolute value of input signal of detector < threshold, output is 0 if input signal of detector ≤ threshold, output is -1. Accordingly, both lead and lag pulses are filtered by the same filter, and there are no separate counters. The correction rate of the filter according to Figure 2b is relatively good, but a problem is a slowly attenuating oscillation in the correction process of the loop. The capacity of a spread spectrum receiver is substantially influenced by the phase accuracy of the spreading code. An error of 0.5 chips, for instance, causes a power loss of 6 dB. Phase tracking accuracy and especially variance therein are affected, except by a code tracking algorithm, by the quality of the loop filter. It has been noticed that in practice the code tracking algorithm operating on the lead/lag principle functions well in receivers. However, the loop filters used therein have drawbacks, and an aim of this invention is to improve such loop filters. When conventional loop filters are used, the variance in phase error increases considerably. An instantaneous error caused by noise is thus big. When a phase step response exceeding a threshold is also taken into consideration, a correction of a phase error may be uncontrolled and the loop is in a continuous correction state. This situation can be corrected by lengthening the integration time of the filter. However, the integration time cannot be lengthened endlessly, because, due to the Doppler effect, the receiver is then no longer capable of tracking the variance in a received signal. Figure 3a shows an output signal (I) of an integrator of a traditional loop filter as a function of time in a situation when a clock signal deviates by two samples from the correct value. In Figure 3b, respectively, a signal is at the output of a threshold detector of the loop filter. In the conventional loop filter, exceeding the threshold takes place in the output signal of the integrator, whereby the phase is corrected by one sample too much. Then the loop receives a control signal of opposite sign and the integrator integrates in the opposite direction until the threshold is exceeded. When a negative threshold is exceeded, a negative clock control pulse is generated. Such a to-and-fro oscillation may last even for a long time in the presence of noise. European patent application no. 571853 describes a further phase-locked loop control method, according to the preamble of claim 1. In a first aspect of the present invention there is provided a method of controlling a loop filter in a digital phase lock circuit, said circuit having a phase comparator for outputting a phase signal proportional to phase error, said loop filter having a predetermined bandwidth, said loop filter having at least one integrator for providing an output signal responsive to the phase signal, said method comprising: receiving a phase signal from the phase comparator; adjusting the loop filter in a non-linear manner in response to the phase signal that is output from the phase comparator so changing the bandwidth of the loop filter in response, characterised in that the non-linear adjustment of the loop filter comprises deducting a predetermined value from the value contained in one or several integrators of the loop filter, if the phase error exceeds a first threshold or a second threshold. In a second aspect of the present invention there is provided apparatus for controlling a loop filter of a digital phase lock circuit, which phase lock circuit comprises a phase comparator, a loop filter, a clock, a local oscillator and a divider, characterised in that the loop filter comprises a first integrator, having an input coupled to a signal (39) indicative of a phase difference, a first threshold detector, operatively connected to an output of the first integrator and its output being coupled to the clock, and a second integrator having an input coupled to the signal indicative of a phase difference through a first means for providing an absolute value, a second threshold detector operatively coupled to an output of the second integrator and an OR gate, the inputs of which are the output of the second threshold detector and the output of the first threshold detector through a second means for providing an absolute value, and the output of the OR gate is coupled to one of the inputs of the integrators, and which output signal deducts a predetermined value from the content of the integrator. An advantage of the present invention is that there is provided a loop filter by means of which the drawbacks of the previous solutions can be ameliorated. In embodiments in accordance with the invention, oscillation in an adjusting process can be reduced and erroneous corrections caused by noise decreased. Additionally, the implementation of a filter in accordance with the invention is simple. Another advantage of embodiments in accordance with the invention is that the capacity of the loop is improved, particularly by low signal-to-noise ratios, whereby erroneous corrections occur particularly in conventional methods. For high-level phase difference signals, the integration time of the filter becomes longer on account of the non-linear control of the filter. By means of a loop filter in accordance with the invention, second order threshold exceeding of the phase step response of the phase lock can be reduced considerably without the operation of the loop being deteriorated. In the following, the invention will be described in greater detail, by way of example only, and with reference to the attached drawings, in which: Figure 1 shows a digital phase lock structure described above; Figures 2a and 2b illustrate loop filters according to the prior art described above; Figures 3a and 3b illustrate an example of the operation of a loop filter according to the prior art described above; Figure 4a shows an example of an output signal of an integrator used in a loop filter in accordance with the invention as a function of time; Figure 4b shows an example of an output signal of a threshold detector used in the loop filter in accordance with the invention as a function of time; Figures 5a and 5b show two possible embodiments of the loop filter implementing the principle in accordance with to the invention; and Figure 6 illustrates the structure of the integrator used in the filter in accordance with the invention. The operation of a loop filter in accordance with the invention is illustrated in Figures 4a and 4b. Figure 4a shows an output signal (I) of an integrator used in the loop filter in accordance with the invention as a function of time in a situation when a clock signal deviates by two samples from the correct value. In Figure 4b, respectively, a signal is at the output of a threshold detector of the loop filter. The signal entering the loop filter increases the signal at the output of the integrator, and when a threshold level K is exceeded (or passed under, on the negative side), a phase correction is performed. In a method in accordance with the invention, a certain predetermined number is deducted from the content of the integrator in connection with a phase correction, due to which the output signal of the integrator falls abruptly. In the case shown in Figure 4, the incoming signal after a first correction pulse increases further the value at the output of the integrator, and a second correction pulse is generated when the threshold is achieved again. A certain predetermined number is again deducted from the content of the integrator, and no extra correction takes place thereby. By such a non-linear control, the oscillation in the adjusting process can be reduced and phase corrections caused by noise can be decreased. Consequently, the bandwidth of the filter in accordance with the invention changes automatically because of the non-linear control to be performed on the basis of the signal from the phase comparator. Figure 5a shows a block diagram of an example of a digital loop filter implementing a method in accordance with the invention. The filter comprises a first integrator 41, to the input of which is connected a signal 39 coming from the phase comparator. The output of the integrator is connected to the input of a first threshold detector 43, the output of which threshold detector is coupled to a clock of a phase lock circuit and by which signal the clock is either advanced or retarded. The filter comprises additionally a second integrator 42, to the input of which is connected the signal from the phase comparator through a first rectifier 40 for providing absolute value signals. The output of the second integrator 42 is coupled as an input to a second threshold detector 44. The filter comprises further an OR gate 46, the input of which consists of the output of the second threshold detector 44 and the output of the first threshold detector 43 through a second rectifier 45 for providing absolute value signals. An output signal 47 of the OR gate 46 is connected to the control input of the integrators 41, 42. The first integrator 41 is used for filtering and detecting a timing error signal. In the case when threshold K is exceeded, the phase of the phase lock is corrected according to the output signal of the threshold detector so as to advance or retard the phase. The second integrator 42 is used for detecting the zero value of the timing error signal, i.e. for preventing faulty corrections caused by noise. When threshold K or K' of the threshold detector 43 or 44 is exceeded, a predetermined constant value is deducted from the content of the integrators 41 and 42 by means of the signal 47 connected to the control input. To make the filter operate in a desired manner, which means that threshold K is exceeded before threshold K', for a desired correction of the phase, the integration times T and T' of the integrators 41 and 42 shall be selected in such a way that T'>T. The capability of functioning can also be secured by a suitable selection of the thresholds K and K' of the threshold detectors 43 and 44. The purpose of the first rectifier 40 for providing an absolute value signal is to change the negative signal values to positive ones and thus to eliminate errors caused by noise. In case the signal contains only noise (which means that no phase error exists), negative and positive signal values are equally probable. The second rectifier 45 for providing absolute value signals is needed in order that lead and lag signals starting from the threshold detector 43 would be processed in the same way in the OR gate 46. Figure 5b shows a block diagram of another example of a digital loop filter implementing a method in accordance with the invention. The filter comprises an integrator 48, to the input of which is connected a signal 39 from the phase comparator. The output of the integrator is connected to the input of a threshold detector 49, the output of which is coupled to the clock of the phase lock circuit and by which signal the clock is either advanced or retarded. Additionally, the output of the threshold detector is coupled to a second input of the integrator 48 to provide a constant value to be deducted from the content of the integrator 48. In comparison to the preceding embodiment, the structure and implementation of this filter is simpler and cheaper. As to the capacity, the filter of this type is, however, somewhat inferior to the loop filter according to the preceding embodiment. One possible structure of an integrator to be used in both above-described filters in accordance with the invention is illustrated in Figure 6. The integrators 41 and 42 mentioned in the previous figure can be implemented by means of an integrator according to the Figure 6. The integrator comprises an amplifier 53, a delay element 51, a first feedback circuit 55, the input of which is the output of the delay element 51, a switch 52, the input of which is the output of the delay element as well as a signal 47 coming to the control gate of the integrator. The integrator further comprises a second feedback circuit 54, the input of which is the output of the switch 52, and a summer 50, the positive inputs of which are the signal coming to the integrator through the amplifying means 53 and the output of the feedback circuit 55 and the negative input of which is the output of the second feedback circuit 54. The feedback circuit 55 and the amplifier 53 affect the bandwidth of the integrator in a normal manner. In accordance with a preferred embodiment of the invention, the threshold value K or K' of the loop filter having been exceeded, the signal 47 comes to the control gate of the integrator through an OR gate, under control of which signal a value determined by the feedback circuit 54 is deducted from the output of the integrator. Though the invention has been described above referring to the examples according to the attached drawings, it is clear that the invention is not restricted to those, but can be varied in many ways within the scope of the inventive idea set forth in the attached claims. For example, It is possible to implement the loop filter by means of an appropriately conditioned signal processor, operable under the control of software.

Claims: A method of controlling a loop filter (11) in a digital phase lock circuit, said circuit having a phase comparator (10) for outputting a phase signal proportional to phase error, said loop filter (11) having a predetermined bandwidth, said loop filter (11) having at least one integrator (41, 42, 48) for providing an output signal responsive to the phase signal, said method comprising: receiving a phase signal from the phase comparator (10); adjusting the loop filter (11) in a non-linear manner in response to the phase signal that is output from the phase comparator (10) so changing the bandwidth of the loop filter (11) in response, characterised in that the non-linear adjustment of the loop filter (11) comprises deducting a predetermined value from the value contained in one or several integrators (41, 42, 48) of the loop filter (11) if the phase error exceeds a first threshold (43) or a second threshold (44). A method according to claim 1, characterised in that the bandwidth of the loop filter decreases when the difference signal coming from the phase comparator is large. Apparatus for controlling a loop filter of a digital phase lock circuit, which phase lock circuit comprises a phase comparator (10), a loop filter (11), a clock (12), a local oscillator (14) and a divider (13), characterised in that the loop filter comprises a first integrator (41), having an input coupled to a signal (39) indicative of a phase difference, a first threshold detector (43), operatively connected to an output of the first integrator (41) and its output being coupled to the clock (12), and a second integrator (42) having an input coupled to the signal (39) indicative of a phase difference through a first means (40) for providing an absolute value, a second threshold detector (44) operatively coupled to an output of the second integrator (42) and an OR gate (46), the inputs of which are the output of the second threshold detector and the output of the first threshold detector through a second means (45) for providing an absolute value, and the output (47) of OR gate (46) is coupled to one of the inputs of the integrators (41, 42), and the output signal (47) deducts a predetermined value from the content of the integrator (41, 42). Apparatus according to claim 3, characterised in that the integrator (41, 42, 48) comprises an amplifying means (53), a delay element (51), a first feedback means (55), the input of which is the output of the delay element (51), a switching means (52), the inputs of which are the output of the delay element and the second input (47) of the integrator, a second feedback means (54), the input of which is the output of the switching means (52), and a summer (50), the positive inputs of which are the signal coming to the integrator through the amplifying means (53) and the output of the first feedback means (55) and the negative input of which is the output of the second feedback means (54), and the output of which summer (50) is connected to the input of the delay element and in which summer (50) a constant value determined by the feedback means (54) is deducted from the content of the integrator under control of the signal (47) coming to the second input of the integrator. Apparatus according to claim 3 or 4, wherein the predetermined value is a constant.

Inventors: LATVA-AHO MATTI, LATVA-AHO, MATTI
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1994
Main classification: H03L7
IPC codes: H03L7